add hide icon sample on showLine

components/tree/index.en-US.md

@@ -78,3 +78,9 @@ to avoid double computing. But, this brings some restrictions.
   ? <Tree>{this.state.treeData.map(data => <TreeNode />)}</Tree>
   : 'loading tree'}
 ```
+
+## FAQ
+
+### How to hide file icon when use showLine?
+
+File icon realize by using switcherIcon. You can overwrite the style to hide it: https://codesandbox.io/s/883vo47xp8

components/tree/index.zh-CN.md

@@ -76,3 +76,9 @@ subtitle: 树形控件
   ? <Tree>{this.state.treeData.map(data => <TreeNode />)}</Tree>
   : 'loading tree'}
 ```
+
+## FAQ
+
+### 在 showLine 时，如何隐藏子节点图标？
+
+文件图标通过 switcherIcon 来实现，如果不需要你可以覆盖对应的样式：https://codesandbox.io/s/883vo47xp8